Tile backends currently assert in their command handlers that the
input command is between 0 and GEGL_TILE_LAST_COMMAND (this is true
for the built-in backends, but we can assume it's also true for
custom backends.)  This prevents us from adding new tile commands
without breaking the ABI.

Instead, add a new gegl_tile_backend_command() function, which acts
as a default command handler for tile backends, and to which their
handlers should forward unhandled commands (this function currently
simply performs the range check for the command -- however, against
the GEGL_TILE_LAST_COMMAND value of the runtime GEGL -- and returns
NULL; we can add differet default behaviors for different commands
as necessary.)

In order to remain backward compatible with tile backends compiled
against older versions of GEGL, which still contain the above
assertion, we replace the subclass's command handler with a thunk
upon construction, which tests whether the original handler
forwards unhandled commands to gegl_tile_backend_command(), and re-
replaces the handler with either the original handler if it does,
or a compatibility shim, which only forwards pre-0.4.10 commands to
the original handler, if it doesn't.